In graphics display terminals, it is known to provide a plurality of memories for various display fields. One such memory is termed a viewport memory and contains the storage necessary to hold a pixel image for one "screen" of the display. Another such memory is termed the "world-plane" memory and contains sufficient storage for all pixels in a complete graphics image. Ordinarily, the world-plane image is considerably larger than the image displayed through the viewport memory, with the viewport only showing a portion of the world-plane. Under user control of a keyboard or other instrumentality, the viewport may be shifted from place to place within the world-plane to display images contained in the world-plane.
In the U.S. Pat. No. 4,720,703 to Schnarel, Jr. et al, an apparatus and method for panning a viewport image across a world-plane image is disclosed. Schnarel et al employ a cursor to enable the panning of the viewport relative to the world-plane image. As long as the cursor is within the viewport, it is employed in the standard manner. When, however, the cursor is moved outside the viewport, the viewport is panned to include the cursor. By "panning" is meant moving the viewport in a measured manner to cause it to scan from one location to another with the cursor just being brought within the viewport before the panning ceases.
In present-day graphics systems, Zoom features are included which enable the "blow-up" of discrete areas of the world-plane image. When in the Zoom mode, the panning of the viewport across a world-plane image may consume considerable time. Even in non-Zoom displays, it is often desirable to rapidly pan the viewport to enable rapid examination of various parts of the world-plane image, while also providing slower pan rates for detailed examination of the image. Thus, the viewport should be made "pannable" at different rates of speed, depending upon the user's desire. It is further preferable that the panning control be as simply implemented as possible to enable user-friendly operation of the display function.
Accordingly, it is an object of the invention to provide a system and method for panning a viewport at variable speeds across a world-plane image.
It is another object of this invention to enable a variable speed pan feature for a display wherein a single cursor control device also controls the panning action.